Numerous process for the separation of air into its constituent components are known in the art, among these are the following:
U.S. Pat. No. 3,210,951 discloses a fractionation cycle employing first and second fractionating zones operating under different pressures and including two reboiler/condensers. Both of the reboiler/condensers are interconnected with the stages of fractionation in such a manner as to effect the required reboil and reflux production with minimum pressure differential between the stages of rectification and also decreased the irreversibility of the overall fractionation process thereby obtaining the desired separation with the high pressure stage operating under substantially reduced pressure.
U.S. Pat. No. 3,277,655 discloses an improvement to the fractionation process taught in U.S. Pat. No. 3,210,951. In this process, the heat exchange occurring in one of the two reboiler/condensers between the bottoms liquid from the lower pressure column and the gaseous material from the high pressure column results in complete vaporization of the liquid from the low pressure column thereby satisfying the reboiler requirements of the low pressure column. Additionally, when the liquefied gaseous material from the high pressure column is introduced into the lower pressure column it improves the reflux ratio in the upper portion of the low pressure column which increases the separation efficiency and makes it possible to lower the pressure of the gaseous mixture entering the cycle.
U.S. Pat. No. 3,327,489 discloses another improvement to U.S. Pat. No. 3,210,951 to lower the pressure in the high pressure fractionator. In the process, the pressure reduction is obtained along with the associated power reduction by establishing a heat exchange between gaseous material, which may comprise the feed mixture, and a liquid component collecting in the bottom of the low pressure fractionator, with the liquid component being under different pressure.
U.S. Pat. No. 3,754,406 discloses a process for the production of low purity oxygen, in which a low pressure stream of incoming air is cooled against outgoing gas streams and fed into a high pressure distillation column. A high pressure stream of incoming air is cooled against outgoing gas streams, partially condensed against boiling oxygen product in a product vaporizer, and separated into gas and liquid streams. The liquid stream being subcooled and expanded into a low pressure fractionating column. The gas stream is reheated and expanded to provide process refrigeration and is introduced into the low pressure fractionating column. Crude liquid oxygen from the bottom of the high pressure column is cooled and introduced into the low pressure column after being used to liquefy some of the nitrogen from the high pressure column in an external reboiler condenser. Liquid oxygen product from the low pressure column is pumped to a higher pressure before being passed to the subcooler and the product vaporizer. The remainder of the high pressure nitrogen is liquefied in a second external reboiler/condenser and is used as reflux for the two columns. A waste nitrogen stream is removed from the low pressure column.
U.S. Pat. No. 4,222,756 discloses a process in which a two pressure distillation column is used in which both pressurized column sections are refluxed with a nitrogen-enriched stream. The low pressure column is fed by an oxygen-enriched stream from the high pressure column which is expanded to reduce its pressure and temperature.
U.S. Pat. No. 4,453,957 discloses a cryogenic process to efficiently produce large quanties of nitrogen at elevated pressure by use of a double column and multiple condenser-reboilers.
U.S. Pat. No. 4,464,188 discloses a process and apparatus for the separation of air by cryogenic distillation in a rectification column using two nitrogen recycle streams and a sidestream of the feed air stream to reboil the column. One of the nitrogen recycle streams is expanded to provide refrigeration and to provide power to compress the feed air sidestream.
U.S. Pat. No. 4,617,036 discloses a process for the cryogenic distillation of air to recover nitrogen in large quantities and at relatively high pressure, wherein a portion of the nitrogen reflux for the distillation is achieved by heat exchanging nitrogen gas in a side reboiler against waste oxygen at reduced pressure.
U.S. Pat. No. 4,702,757 discloses a process utilizing high and low pressure distillation columns for the production of an oxygen-enriched air product. Feed air is fed to the main heat exchangers at two pressures. The high pressure feed air from the main exchanger is used to supply refrigeration, by expanding a portion of the high pressure air prior to introducing that portion into an intermediate location in the low pressure column, and to vaporize the oxygen-enriched air product prior to using the stream as reflux for the high pressure column. The low pressure feed air from the main heat exchangers is partially condensed to supply reboiler duty to a low pressure column and is then fed to a high pressure column. The high pressure column condenser is used to reboil an intermediate liquid in the low pressure column.
U.S. Pat. No. 4,704,147 discloses a process for the production of an oxygen-enriched air product, feed air is fed to the main heat exchangers at two pressures. The high pressure feed air from the main exchanger is partially condensed to vaporize the oxygen-enriched air product. This partially condensed feed air is separated with the vapor phase being warmed and expanded to supply refrigeration and subsequently being fed to the low pressure fractionation section, and the liquid phase being used to reflux both the high pressure and low pressure fractionation sections of a double distillation column. The low pressure feed air from the main heat exchangers is fed to the high pressure fractionation section. The high pressure fractionation section condenser is used to provide reboiler duty to the low pressure fractionation section.
U.S. Pat. No. 4,704,148 discloses a process utilizing high and low pressure distillation columns, for the separation of air to produce low purity oxygen and waste nitrogen streams. Feed air from the cold end of the main heat exchangers is used to reboil a low pressure distillation column and to vaporize the low purity oxygen product. This heat duty for column reboil and product vaporization is supplied by splitting the air feed into at least three substreams. One of the substreams is totally condensed and used to provide reflux to both the low pressure and high pressure distillation column, preferably the substream which is fed to the oxygen vaporizer, while a second substream is partially condensed with the vapor portion of the partially condensed substream being fed to the bottom of the high pressure distillation column and the liquid portion providing reflux to the low pressure column. The third substream is expanded to recover refrigeration and then introduced to the low pressure column as column feed. Additionally, the high pressure column condenser is used as an intermediate reboiler in the low pressure column.
Patent Application U.S. Ser. No. 07/132,534 filed Dec. 14, 1987, now U.S. Pat. No. 4,738,210 discloses a process to produce large quantities of pure nitrogen and small amounts of high purity oxygen co-product which utilizes a modified single distillation column nitrogen generator. The modification is the addition of a small second column which purifies a portion of the oxygen enriched liquid from the nitrogen generator overhead condenser. Reboiling for the second column is provided by condensing part of the nitrogen overhead from the nitrogen generator. This condensed nitrogen is used as reflux in the nitrogen generator.